U14: Field Testing & Analysis of Braking Performance of In-Service Trucks

Shaffer, Steven J.; Long, Amy M. · 2009 · ROSA P / National Transportation Research Center, Incorporated, University Transportation Center

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Summary

This study addresses the lack of empirical data regarding the braking performance of in-service commercial motor vehicles (CMVs) since the early 1980s. The research was motivated by safety concerns regarding whether current visual inspection protocols, specifically the Commercial Vehicle Safety Alliance (CVSA) Level I inspection, accurately reflect actual stopping capabilities. A primary concern was the potential for degraded performance due to unregulated aftermarket replacement brake components, which may not be detectable through visual inspection alone. The project aimed to collect a high-quality dataset to evaluate current vehicle performance and assess the correlation between visual inspections and actual braking metrics. The methodology involved testing 82 CMVs using three distinct assessment methods: a 20-mph stopping distance test, a Performance-Based Brake Tester (PBBT) measurement, and a CVSA Level I visual inspection. Data collection occurred in two phases: 59 vehicles from cooperative local fleets in Tennessee and 23 randomly selected vehicles in New Mexico. Stopping distances were measured using a GPS-based data acquisition system (V-Box III), which recorded position, speed, and application air pressure. The PBBT measured individual brake forces and wheel loads to calculate braking efficiency. Visual inspections were conducted by certified inspectors following standard out-of-service criteria, supplemented by specific component checks such as push-rod stroke measurements. The results indicated that 16% of the tested vehicles (13 of 82) failed to meet the minimum stopping distance requirement of 40 feet from 20 mph. Only 8.5% (7 vehicles) failed the PBBT criterion, which requires a total braking force of at least 43.5% of the gross vehicle weight. Notably, only three vehicles failed both tests, suggesting that driver technique significantly influences stopping distance results. Comparative analysis revealed that the randomly selected vehicle population performed worse than the cooperative fleet vehicles; the mean stopping distance for random vehicles was 38.0 feet, compared to 34.4 feet for fleet vehicles. Similarly, the mean braking efficiency (BFtot/GVW) for random vehicles was 48.0%, significantly lower than the 56.1% observed in fleet vehicles. This disparity is attributed to the likely superior maintenance standards of volunteer fleets. The study concludes that while the project successfully demonstrated the feasibility of collecting comprehensive brake performance data, the discrepancy between stopping distance and PBBT results highlights the influence of driver behavior on performance tests. The findings suggest that visual inspections may not fully capture braking deficiencies caused by aftermarket parts, and that different assessment methods may not be equivalent for regulatory purposes. The archived data, including air pressure signals, provides a foundation for future analyses to quantify driver influence and refine safety regulations.

Key finding

16 percent of tested commercial motor vehicles failed to meet the minimum 20-mph stopping distance requirement, and only three of the vehicles that failed the PBBT braking force criterion also failed the stopping distance test.

Methodology

field_study

Sample size: 82

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• braking response